1. Field of the Disclosure
The disclosure relates to the field of electromagnetic induction well logging. More specifically, the present disclosure is a method of and an apparatus for making deep resistivity measurements ahead of the drill bit using transient electromagnetic signals.
2. Description of the Related Art
In transient electromagnetic (TEM) methods, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. Electric currents diffuse outwards from the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor predominantly from different investigation depths. Particularly, at a sufficiently late time, the transient electromagnetic field is sensitive mainly to remote formation zones and only slightly depends on the resistivity distribution in the vicinity of the transmitter.
It is highly desirable to have short spacing MWD/LWD system with minimum number of sensors for deep ahead of the bit resistivity measurements due to multiple operational reasons. Unfortunately, conventional induction low frequency measurements can provide the increased depth of investigation only with increasing the transmitter/receiver spacing. The obvious price for the increased depth of investigation is the significantly lower signals due to the lower frequency and larger spacing. For the ahead of the bit measurement there is another disadvantage of using long spacing—moving the sensors away from the target reduces the sensitivity of the measurement to the ahead of the bit target.
The present disclosure is directed to making deep (greater than 10 meters) transient electromagnetic (DTEM) measurements using a short spacing of the transmitters and receivers that has real-time interpretation capability.